The invention relates to a scandate cathode having a cathode body comprising a matrix of at least a high melting-point metal and/or alloy and a barium compound in contact with the matrix material which can supply barium to the emissive surface by chemical reaction with the matrix material.
The invention also relates to methods of manufacturing such a cathode and to an electron beam tube comprising such a cathode.
Cathodes of the type mentioned above are described in the article "Properties and Manufactured Top Layer Scandate Cathodes", Applied Surface Science 26 (1986), 173-195, J. Hasker, J. v. Esdonk and J. E. Crombeen. In the cathodes described in this article scandium oxide (Sc.sub.2 O.sub.3) grains of several microns or tungsten (W) grains which are partially coated with either scandium (Sc) or scandium hydride (Sc H.sub.2) are processed at least in the top layer of the cathode body. The cathode body is manufactured by pressing and sintering tungsten grains, whereafter the pores of the sintered body are impregnated with barium-calcium-aluminate. By chemical reaction with the tungsten of the matrix during operation of the cathode, the barium-calcium-aluminate supplies barium to the emissive surface in order to maintain the electron emission.
To be able to realize a very high cathode load after assembly in, for example, a cathode ray tube and activation of the cathode, it is important that a scandium-containing layer having a thickness of some monolayers has formed on the cathode surface during impregnation by reaction with the impregnant. To this end the impregnation process must be performed very carefully. As compared with the processing of an impregnated tungsten cathode, which may be coated with, for example osmium, this may be considered a drawback.
As has been proved by experiments described in the above-mentioned article, an ion bombardment which may occur in practice, for example during the manufacture of television tubes, may entirely or partly remove the scandium containing layer, with attendant detrimental results for emission. Since Sc.sub.2 O.sub.3 is not very mobile (in the cathodes manufactured using W partially coated with Sc or Sc H.sub.2 oxidation occurs during impregnation), the said scandium-containing layer cannot be fully regenerated by reactivating the cathode. As compared with an impregnated tungsten cathode, this may also be considered a drawback.